Processes for developing electrostatic images using dry toner are well known in the art. Such development systems are used in many electrographic or electrophotographic printers and copiers (collectively referred to herein as “printers”) and typically employ a developer consisting of toner particles, hard magnetic carrier particles and other components. Magnetic brush development may also be used for powder coating applications. In many current and prior art developers, the carrier particles are much larger than the toner particles, on the order of up to 30 times larger.
The developer is moved into proximity with an electrostatic image carried on a receiver, whereupon the toner component of the developer is deposited on the receiver. Deposition of toner onto the receiver is driven by the electric field between the electrostatic image on the receiver and the magnetic brush. In electrophotographic printers the receiver is a photoconductor, and the toner is subsequently transferred to a sheet of paper or other final receiver to create the final output of the device, which can be an image. Developer is moved into proximity with the electrostatic image by a rotating toning shell, an electrically-biased, conductive metal roller that is rotated cocurrent with the photoconductor, such that the opposing surfaces of the photoconductor and toning shell travel in the same direction. For powder coating applications, an electric field drives toner deposition from the developer onto a substrate or receiver, which can be electrostatically charged or to which a bias voltage can be applied relative to the toning shell.
Located inside the toning shell is a multipole magnetic core, having a plurality of magnets, that is either fixed relative to the toning shell or that rotates, usually in the opposite direction of the toning shell. The developer is deposited on the toning shell and the toning shell rotates the developer into proximity with the receiver, at a location where the receiver and the toning shell are in closest proximity, referred to as the “toning nip.”
On the toning shell, the magnetic carrier component of the developer forms a “nap,” similar in appearance to the nap of a fabric, because the magnetic particles form chains of particles that rise vertically from the surface of the toning shell in the direction of the magnetic field. The nap height is maximum when the magnetic field from either a north or south pole is perpendicular to the toning shell. Adjacent magnets in the magnetic core have opposite polarity and, therefore, as the magnetic core rotates, the magnetic field also rotates from perpendicular to the toning shell to parallel to the toning shell.
When the magnetic field is parallel to the toning shell, the chains collapse onto the surface of the toning shell and, as the magnetic field again rotates toward perpendicular to the toning shell, the chains also rotate toward perpendicular again. Thus, the carrier chains appear to flip end over end and “walk” on the surface of the toning shell and, when the magnetic core rotates in the opposite direction of the toning shell, the chains walk in the direction of photoconductor travel.
The magnetic toning roller and toning shell are located in a housing or sump, in which fresh toner is mixed with the magnetic developer and applied to the toning shell. Augers and ribbon blenders are typically used to mix developer and fresh toner. Bucket assemblies, magnetic rollers internal to the sump or paddle wheels are used to apply the developer to the toning shell. The amount of developer applied to the toning shell is usually limited by a skive adjacent to the toning shell, which extends down the length of the toning shell and is spaced uniformly from the toning shell.
Exemplary development systems that implement hard magnetic carriers are described in U.S. Pat. Nos. 4,473,029 and 4,546,060, the contents of which are incorporated by reference as if fully set forth herein. Alternatively, in other development systems the toning shell may or may not rotate. Other developments systems implement magnetic carriers that are not hard (i.e. soft). Still alternatively, in other development systems the magnetic core may not rotate.
Ideally the toning shell and the magnetic core are as long as necessary to provide the developer to the entire printing width of a printer. In the case of coating applications, the toning shell and the magnetic core are as long as necessary to provide developer to the entire width of the substrate. The toning width could be increased by increasing the length of the toning station; however, the overall length of the toning station described above is limited by manufacturing tolerances for these components and the adverse effects these longer components have on image quality. Thus, increasing the length of the toning station causes a corresponding decrease in the uniformity of the toner deposition and the image quality of a printer or other powder deposition device.
Many common printing applications and powder coating applications require a larger toning width. For these applications there exists a tradeoff between increasing the length of the toning roller and maintaining the uniformity of toner deposition, and therefore also the uniformity of laydown in a coating system or the image quality in a printer. That is, the toning width could be increased by increasing the length of the toning roller, but runout, spacing differences to the receiver along the length of the roller or other non-uniformities in the longer toning roller would cause a decrease in the uniformity of toner deposition and the image quality. Similarly, the uniformity of toner deposition and the image quality could be maintained by using a shorter toning roller with more precise dimensions, which would then decrease the toning width of the development station.
Therefore, there exists a need to provide an improved system for increasing the toning width of a printer or other toner deposition system while still maintaining the quality of the resulting image and the uniformity of toner deposition.